Creative methods to improve vertical wafer boats by way of their support structure, capacity, loading capability, and manufacture demand attention. Semiconductor wafers are put into heat treatment furnaces for various treatments involving, for example, oxidation, diffusion and thin film deposition. A wafer boat is used as a support mechanism for conducting such processes. The boat is robotically loaded with wafers by a transfer fork. Then, the boat is vertically placed into a furnace while support grooves within the members of the boat hold the wafers. One treatment in particular involves low-pressure chemical vapor deposition (LPCVD) that results in a thin film deposit throughout the semiconductor wafers and the internal surfaces of the wafer boat. The manufacturing process of wafer boats is relevant to such a deposition process because it defines the contact points to the wafers and structure for intense treatments.
FIG. 1 depicts a cross-sectional view of a simplified prior art wafer boat. Three semiconductor wafers sit inside support grooves illustrated by two of possibly several support members in the boat. As known by those skilled in the art, the wafer boat 10 comprises a top plate 12 and a bottom plate 14 vertically opposing one another and up to six rectangular, triangular or circular-shaped support members 16. The support members 16 connect the top and bottom plates 12, 14 to form the boat structure 10. As illustrated, protrusions 18 commonly referred to as dividers or teeth, are created that support the silicon wafers 20 at their posterior surface 22. The boats 10 are commonly made of quartz or high-purity silicon carbide and the wafers 20 are separated in a vertical direction by a distance “D” dictated by a vertical spacing of the protrusions or teeth 18. The teeth 18 present here are of an inclined ramp shape with the bottoms essentially perpendicular to the vertical support member 16.
Support teeth 18 for the wafers 20 should be as long as possible to deepen the support grooves and to prevent the occurrence of dislocation. In addition, the distance “D” between adjoining support grooves is preferably small in order to simultaneously treat as many semiconductor wafers as possible. A diamond blade or saw is necessary to carry out groove machining with high accuracy in prior art manufacturing methods. Groove machining becomes more difficult with deeper grooves and with smaller distances between adjoining grooves. Consequently, the probability increases of fractured teeth during manufacture and lower product yield.
The wafer boats require occasional cleaning by an acid in order to lower the incidences of contamination by impurities from within the boats. When the supporting grooves are deep, and the distance between adjoining supporting grooves are narrow, the supporting pieces may fracture during the cleaning operation.
In addition, particulates associated with the wafer boat that may cause defects tend to take place in the vicinity of the wafers during furnace processing. Therefore, the groove design and method by which the wafers are supported can impact the effects of such defects on the wafers. During furnace processes, such as LPCVD, a thin film deposits over the semiconductor wafer as well as the inside of the wafer boat. Afterwards, removal of a wafer generates particulates capable of forming defects on the front surface of the wafers. After several deposition processes, the wafer boat itself can generate particulates. Points of stress from buckling of deposition build-up within support grooves can generate particulates that contaminate the front active side of the wafer. This necessitates a better groove design to alleviate such points of stress and increase the longevity of the wafer boat.
With the conventional wafer boat design of prior art FIG. 1, micro-scratches and chipping particle formation is difficult to avoid as a result of the necessary contact surface between the silicon wafer and support teeth. Mechanical contact between the wafer 20 and the wafer boat teeth 18 create micro-scratches and chipping particles at the wafer backside during loading and unloading operations because of the inaccuracy of robotic placement into such narrow grooves. Because micro-scratches and chipping particles may contribute to chip defects being produced, manufacturing yields may decrease. Removal of the wafers in a systematic manner can prevent the particulates from contaminating wafers below. Robots normally remove wafers in batches of five very rapidly, so removing wafers from the bottom first prevents substantial cross contamination from other wafers.
Furthermore, each contact point 24 generates a cold zone or temperature nonuniformity associated with the wafer that can contribute to a substantial degradation of the thickness uniformity of the deposited layer. Consequently, there is a need for improved wafer boat design.